An automatic transmission for an automobile comprises a torque converter, a wet clutch, a gearbox, and a hydraulic control mechanism for controlling them. In such an automatic transmission where the power from an engine is transmitted through the torque converter as a fluid joint, the transmission loss caused by the difference between the input and output rotations of the torque converter is regarded as the greatest cause of decrease in fuel consumption rate. Accordingly, as a means of improving fuel consumption rate a torque converter with built-in lock-up clutch having a high transmission efficiency has been adapted to reduce the transmission loss, and a slip control system has been introduced to extend its operating area. Since the slip control adapts a system of sliding the lock-up clutch while monitoring the engine rotating speed and the torque converter output rotating speed and controlling its relative slip velocity by the hydraulic mechanism, the lock-up clutch can be operated even in a low velocity area where the direct connection was difficult in the past.
However, it has been recognized that an automatic transmission oil having a performance qualitatively different from the automatic transmission oil used for automatic transmissions equipped with no slip control mechanism is required to make the above automatic transmission smoothly function. Namely, although to increase the torque capacity was the important subject in the past, it is indispensable for the slip control system to be provided with shudder preventing performance, and the transmission torque capacity to wet friction material necessary for reduction in size and weight is also required.
Accordingly, various friction modifiers, for example, phosphate, fatty acid amide and the like have been proposed therefor. However, the transmission torque capacity and the shudder preventing performance are in a trade-off relationship, and in the incorporation with such a fiction modifier, a weak point has been pointed out that the friction coefficient in low slip velocity area of the lock-up clutch part of the automatic transmission is reduced, so that a sufficient transmission torque capacity cannot be provided in the engagement of the lock-up clutch, even if the shudder preventing performance can be improved. Therefore, an automatic transmission oil having an organic acid metal salt such as sulfonate or fenate of calcium or the like mixed therein for the purpose of improving the transmission torque capacity has been proposed.
However, an automatic transmission oil related to the above proposal had a problem that the μ(coefficient of friction)-V(slip velocity) characteristic is deteriorated when used for a long time to shorten the durable life of shudder (stick slip) preventing performance. Namely, although an increase in mixing quantity of the friction modifier is effective in for improvement in μ-V characteristic, a problem has been pointed out that the increase in quantity reduces the friction coefficient in low velocity slip range, so that a sufficient transmission torque capacity cannot be provided in the engagement of the lock-up clutch to cause a loss of power transmission energy.